Display system, related display method and computer program

ABSTRACT

The invention relates to a display system included in a device for traveling a path between at least two distinct points and for using at least one heading datum, wherein the display system comprises at least:
         an image sensor configured to acquire environmental images during the path traveled by the device;   an image processing module capable of determining the drift of the device in real time;   a module for determining information that is representative of the route taken by the device,   a module for real-time determination of the true heading followed by the device from the information that is representative of the route and the drift of the device,   a module for returning to the user information that is representative of the true heading.

The present invention relates to a display system included in a devicethat is capable of moving between two distinct points and capable ofusing at least one heading datum, in particular a head-up displaysystem, helmet-mounted head-up or even head down.

The invention also relates to a display method implemented by such adisplay system.

The invention also relates to a computer program comprising softwareinstructions which, when executed by a computer, implement such adisplay method.

The invention offers many applications, in particular the use of a trueheading to improve the view of a user of a device, wherein the user isable to monitor both the environment in which it moves as well asinformation provided by the instruments of the device by means of thedevice or by wearing the device.

In fact, the true heading is a fundamental element for air, sea, spaceor ground navigation when the device is housed or embedded in a vehicle(i.e. the displacement of the device corresponds to that of thevehicle), or for applications in the field of virtual reality oraugmented reality when the device is, for example, an electronic displaysystem comprising an electronic device, for example, a smartphone thatis removable or integrated in a helmet, with or without a displayscreen, and a helmet comprising a support to receive the electronicdevice, a surface bearing against the user's face, facing their eyes,and one or two optical devices arranged between the receiving supportand the bearing surface. In other words, in the latter case of theapplication of virtual or augmented reality, the device is able totravel a route because of its being borne by a user.

It is known to obtain a magnetic heading using a magnetometer that isgenerally integrated in low-cost AHRS (Attitude and Heading ReferenceSystem) and then correct it using a magnetic declination table to derivea true heading.

However, the already poor intrinsic performance of the magnetometer isgenerally degraded after installation in a vehicle capable of using datacorresponding to the heading, such as an aircraft, a ship (submersible,such as a submarine, or non-submersible) or a spatial or terrestrialvehicle.

A heading error of two or three degrees is common. This inaccuracy isgenerally acceptable for an air, sea, space or ground navigationapplication (because it is compensated for by the use of ground beaconsor GPS waypoints to recalibrate the trajectory) or in the case of theconformal projection of a symbology and/or of a synthetic head-downimage.

However, such a heading inaccuracy becomes disruptive for the pilot, oreven a source of error, in the case of the conformal projection of asymbology and/or of a synthetic head-up image superimposed on the movingenvironment, or on the mounted head-up image (which must be coherentwith the external vision of the traveling environment and requires aprecise true heading).

For vehicles, equipped only with an AHRS (Attitude and Heading ReferenceSystem), in particular certain helicopters or other turboprops, theinterest of the projection of head up or helmet-mounted information islimited, because of the inaccuracy of the true heading currentlyobtained.

To remedy this, a first solution is to replace the assembly formed bythe center(s) (or sensor(s)) of the low cost AHRS type and magnetometerby an inertial unit to provide a true heading with the desired accuracydirectly. However, this solution has a prohibitive acquisition cost whencompared with the overall cost of the carrying device in question.

A second solution is to search with the help of a video sensor in theexternal image of the known points of interest (track, relief) andrecalibrate them with a synthetic position calculated from the headingdelivered by the reference station of the low-cost AHRS type, whereinthe recalibration allows the heading to be corrected.

Nevertheless, this second solution requires the use of a precisedatabase containing the position of the points of interest. In addition,the visibility conditions must allow one to see sufficiently far todistinguish such points of interest, which makes this second solutioncomplicated to implement or inoperable in case of poor visibility.

The invention therefore aims to address the problems of optimizing thevisualization of a vehicle user by monitoring both its environment atthe same time as information provided by its onboard instruments.

To this end, the object of the invention is a display system included ina device, wherein the device is able to travel along a path between atleast two distinct points and to use at least one heading datum, whereinthe display system comprises at least:

-   -   an image sensor capable of acquiring environmental images along        the path traveled by the device;    -   an image processing module capable of determining the drift of        the device in real time;    -   a module for determining information that is representative of        the route followed by the device,    -   a module for real-time determination of the true heading        followed by the device on the basis of information that is        representative of the route and of the drift of the device,    -   a module for return to the user of information representative of        the true heading.

According to particular embodiments, the display system comprises one ormore of the following characteristics, taken separately or in anytechnically feasible combination:

-   -   the module for return to the user is a display module that is        able for recalibrating a synthetic image and/or a conforming        symbology in the line of sight of the device associated with the        true heading, wherein the display module belongs to the group        comprising at least:        -   a head-up display module capable of projecting the synthetic            head-up image of the device;        -   a head-up display module capable of projecting the synthetic            head-up image borne by at least one user of the device;        -   a head-down display module capable of projecting the            synthetic head-down image of the device;    -   the image processing module comprises an image analysis tool        capable of automatically determining the drift of the device by        analyzing at least two successive images, wherein the drift is        obtained from the angle of displacement of the pixels between        one image and another, wherein the angle of displacement of the        pixels is associated with the angle between the longitudinal        displacement and the lateral displacement of the device;    -   the image analysis tool is an optical odometer tool;    -   the image sensor is pointed towards the ground;    -   the field of view of the image sensor may be reconfigured        according to the instantaneous roll rate and/or the        instantaneous pitch rate of the device and/or according to the        phase of the journey made by the device;    -   the image sensor is integral with the axes of the device or is        gyro-stabilized;    -   the image processing module comprises an image correction tool        capable of correcting the images acquired before or after their        analysis.

The invention also relates to a display method implemented by thedisplay system according to the present invention, wherein the methodcomprises:

-   -   the acquisition of environmental images along the path traveled        by the device;    -   image processing able to determine the drift of the device in        real time;    -   determination of information representative of the route        followed by the device,    -   real-time determination of the true heading followed by the        device from the information representative of the route and the        drift of the device,    -   the return to the user of information representative of the true        heading.

The invention also relates to a computer program comprising softwareinstructions which, when executed by a computer, implement a method asdefined above.

The invention and its advantages will be better understood upon readingthe following detailed description of a particular embodiment, givensolely by way of a non limiting example, wherein this description ismade with reference to the appended drawings, wherein:

FIG. 1 shows a block representation of the display system according tothe invention;

FIG. 2 shows a flowchart of a display method according to the invention.

In the remainder of the description, the term “substantially” willexpress a relationship of equality plus or minus 10%. In addition, theterm “device” means a device capable of moving between two distinctpoints and that is capable of using at least one heading datum.

For example, such a device corresponds to:

-   -   a vehicle, such as an aircraft, a ship (submersible or        non-submersible), a space vehicle or a land vehicle, or    -   an electronic display system borne by a user along a path        between two distinct positions.

Subsequently, for reasons of simplification, an example is usedaccording to which the device corresponds to an aircraft, such as ahelicopter or a turboprop, wherein such an aircraft is, for example,only provided with a low-cost AHRS (Attitude and Heading ReferenceSystem) and without an inertial unit, or is an electronic display systemborne by a user on board an aircraft (i.e. a pilot or crew member of theaircraft).

With reference to FIG. 1, the display system 10 of such a deviceaccording to the present invention, comprises at least one image sensor12 that is capable of acquiring environmental images along the pathtraveled by an aircraft, an image processing module 14 capable ofdetermining the drift of the aircraft in real time, a module 16 fordetermining information that is representative of the route followed bythe aircraft, a module 18 for real-time determination of the trueheading followed by the device from the information that isrepresentative of the route, and the drift of the device, a module 20for return to the user of information that is representative of the trueheading.

The image sensor 12 is, for example, integrated in a camera. The imagesensor 12 is, for example, associated with an optical objective, forexample a fisheye lens, i.e. covering a visual field with a large angleof view of the order 180° or more. The camera comprises the imagesensor(s) and lens positioned in front of the image sensor(s) so thatthe image sensor(s) capture(s) the images through the lens.

Such an image sensor 12 is capable of being calibrated automatically ormanually in order to compensate for deformations related to the opticallens with which it is associated.

The image sensor 12 is, for example, an infrared (IR) sensor, a radar,or a LIDAR (“light detection and ranging” or “laser detection andranging”), and is capable of delivering images in color (i.e. polychromeimage sensor) or, alternatively, monochrome images.

In one particular aspect, the image sensor 12 is configured to maximizethe vision of the ground overflown by the aircraft.

To do this in the case of a device corresponding to an aircraft, theimage sensor 12 is advantageously pointed towards the ground in order toincrease the probability of detection of the terrain (verticalvisibility is generally better than horizontal visibility).

According to a first variant, the camera comprising the image sensor 12is mounted under the carrier corresponding to the aircraft (i.e. outsidethe cockpit on the lower surface of the cabin of the aircraft). Such aposition of the image sensor 12 maximizes ground vision and relevantpoints of interest overflown by the aircraft.

According to a second variant, the image sensor 12 (i.e. as well as thecamera comprising it) is fixed on the cabin at the level of the cockpitwhile remaining outside it.

According to these two variants, the image sensor 12 is then fixed so asto be integral with the axes of the aircraft.

According to a third variant, the image sensor 12 (i.e. as well as thecamera comprising it) is fixed in the cockpit, for example by means of asuction cup on the windshield. In this case, the camera comprising theimage sensor 12 will not necessarily be oriented along the axes of theaircraft because it is fixed by the pilot without a precise reference.The drift measured by means of the image sensor 12 will thus include abias as a function of the viewing angle of the image sensor 12.

However, as long as the camera remains attached to the reference frameof the aircraft from which the low-cost AHRS 22 calculates the attitudeinformation, it is still possible to recalibrate the head-up/supportedhead-up image with the external environment. To do this, a harmonizationcorrection between the axes of orientation of the camera and the axes ofthe aircraft will, for example, be performed automatically or manuallyduring installation.

According to a fourth variant, the image sensor 12 is mounted on ahelmet worn by the pilot of the aircraft, for example the helmet of ahead-up system capable of projecting a synthetic image, comprisinginformation for assisting the piloting of the aircraft in order to bevisually superimposed on the pilot's visual field of navigation.According to this fourth variant, as detailed below, the imageprocessing module 14 then comprises (an) image correction tool(s)capable of correcting the images acquired by the image sensor 12 throughre-projection of these in a geographical horizontal plane (i.e.re-projection in a predetermined common reference for the processing ofall acquired images). The implementation of such a fourth mountedvariant of the image sensor 12 is advantageous because it takesadvantage of the mobility possibilities of the driver's head whichinstinctively tends to look in the direction of external visual cuesvisible in actual flight conditions.

As an alternative, the image sensor 12 may be gyro-stabilized, whichmakes it possible to overcome any need for re-projection and anyassociated re-projection errors.

According to another variant for aircraft already equipped with at leastone camera comprising an EVS (Enhanced Vision System) image sensor 12,the existing camera is advantageously used for the implementation ofthis invention. Such EVS image sensors 12 are, for example, fixed andmounted in the axis of the aircraft or offset, or are mobile sensors(e.g. optronic ball).

According to another variant, a plurality of image sensors 12 isimplemented, wherein these image sensors 12 are, for example, activatedmanually alternately or simultaneously by the pilot, or automatically ina predetermined manner as a function of the phase of flight of theaircraft and/or trim (angle of attitude of the aircraft).

Furthermore, according to an alternative embodiment, the field of view(FOV) of the image sensor 12 may be reconfigured according to theinstantaneous roll rate and/or the instantaneous pitch rate of theaircraft and/or according to the flight phase (i.e. the travel phase) ofthe aircraft, in order to ensure a sufficient number of common pixelsbetween two images captured successively.

The instantaneous roll rate and/or the instantaneous pitch rate of theaircraft and/or the current phase of flight is, for example, deliveredin real time by the low-cost AHRS 22 to a tool for reconfiguring thedisplay system 10 (not shown) of the field of view obtained from theimage sensor 12. Such a variation of the field of view (FOV) isimplemented, in particular, in order to obtain a succession of imagesthat are sufficiently different from each other to allow matching ofpoints between these images. For example, as the scroll speed is lowerat high altitude, the field of view is reduced to maintain thepossibility of such matching. In other words, such a reconfigurationcorresponds to the implementation of a feedback control according to theinstantaneous roll rate and/or the instantaneous pitch rate of theaircraft.

Moreover, according to one particular aspect, such a visual fieldreconfiguration tool is capable of correcting in real time the potentialoptical distortions of the image sensor provided that they arepreviously stored in a memory of the image sensor that is accessible bythe reconfiguration tool, while the memory is integrated or not withinthe display system 10.

The image processing module 14 that is capable of determining in realtime the drift D of the aircraft, comprises a tool for analyzing theimages delivered (i.e. captured) by the image sensor 12 (not shown).

More precisely, such a tool for analyzing captured images is able toautomatically determine the camera's drift D_(camera) by analyzing atleast two successive images captured, wherein the drift D_(camera) isequal to the angle of displacement of the pixels of one image withrespect to another relative to a predetermined constant axis for all theimages. Then, from the drift D_(camera) of the camera, the drift D ofthe aircraft relative to its target cap C_(C) is obtained by theanalysis tool by applying a correction matrix, referred to as benchmarkharmonization, which makes it possible to pass from the camera referenceframe to the aircraft's reference frame, wherein such a drift D of theaircraft corresponds to the angle between the longitudinal displacementand the lateral displacement of the aircraft, and therefore to the driftD thereof.

In other words, the angle of drift D of the aircraft is, for example,expressed by the following relation: D=tan−1(DEPlatDEPlong).

Such a drift D of the aircraft is therefore suitable to be obtained bythe analysis tool by means of different image processing methods (forexample optical flux processing methods, detection and monitoringmethods for tracking pixels, . . . ) and operation(s) to change thereference frame (i.e. reference harmonization correction) in order topass from the camera reference frame (i.e. image sensor reference frame12) to the reference frame of the aircraft in order to retranscribe thedrift D_(camera) camera (itself obtained in the camera repository) inthe reference frame of the aircraft.

In particular, the image analysis tool is an optical odometry toolcapable of implementing a visual odometry process that is capable ofestimating the rotation and/or the translation of the camera between twoinstants of capturing two images that are acquired successively by theimage sensor 12 as the aircraft is traveling, in order to reconstructthe scene that is representative of the moving environment of the cameraand to position the camera there.

In general, as illustrated in particular by the document “VisualOdometry” by D. Nister et al. CPVR 2004, visual odometry is based on thedetection of points of interest in at least two successive POI (Point ofInterest) images, followed by their pairing and estimation of themovement of these points between the two temporal instants of capture ofthe two successive images in question.

For example, one odometry technique corresponds to the SimultaneousLocalization and Mapping (SLAM) technique as described in particular in“Simultaneous map building and localization for an autonomous mobileroot.” by J. Leonard et al., wherein a particular variant of PTAM(Parallel Tracking And Mapping) is able to estimate the position of thecamera comprising the image sensor 12 in a three-dimensional (3D)environment.

More precisely, the optical odometry tool is first of all suitable forimplementing a filter of the image data delivered by the image sensor12, for example a Kalman filter, a Wiener filter or treatment of waveletimage data.

Then, the odometry tool comprises a point of interest detector, such asa Moravec detector, a Harris detector or a Forstner detector.

Then, to establish the pairing of points of interest of two successiveimages, the odometry tool is able to establish the correspondence ofpoints in two successive images (matching) by using for example:

-   -   Scale-Invariant Feature Transform (SIFT), or    -   an image transformation technique based on Speeded Up Robust        Features (SURF) using an approximation of Haar wavelets, or    -   an alternative image transformation technique, called ORB        (Oriented FAST and Rotated BRIEF) as described in particular in        “ORB: an efficient alternative to SIFT or SURF” by Rublee et al.        based on a feature descriptor derived from accelerated segment        tests (FAST—Features of Accelerated Segment Test) and a binary        visual descriptor, for example BRIEF (Binary Robust Independent        Elementary Features), or an image transformation technique based        on the measurement of optical flows (optical flows or visual        scrolling).

According to another particular aspect, the odometry tool is able totake into account the epipolar geometry of the different successiveimages processed by the image processing module 14 and delivered by theimage sensor 12.

When the image sensor 12 is mounted on a helmet worn by the pilot of theaircraft, the image processing module 14 further comprises (an) imagecorrection tool(s) (not shown) that is/are suitable for correcting theimages acquired before or after their analysis, by applying correctionsthat are configured to act on the attitude and the scale factor appliedduring the image acquisition by the image sensor 12.

In other words, such a correction tool is configured to be connectedupstream or downstream of the analysis tool (i.e. directly on thecomputed drift information D_(camera)) of the image processing modulepreviously described.

More specifically, such an image correction tool is capable ofre-projecting each image acquired by the image sensor in a geographicalhorizontal plane from at least one datum delivered by a sensor externalto the image processing module (14) (i.e. external to the box containingthe correction tool).

In particular, such data belongs to the group comprising at least:

-   -   one or more items of information that is/are representative of        the attitude of the aircraft,    -   one or more items of information that is/are representative of        the height of the aircraft above the ground,    -   an item of information that is representative of the visual        field of the image sensor 12.

More specifically, according to the present example where the device isan aircraft, the information that is representative of the pitchrequired for the projection may be given either by a radio altimeter or,failing that, by the use of synthetic height calculated as Mean SeaLevel (MSL) minus the terrain height stored within a database in anaircraft memory. The MSL altitude may be obtained:

-   -   either by a module 16 for determining information that is        representative of the route followed by the aircraft, namely a        GPS or Gallileo geolocation system of the aircraft,    -   or by corrected air pressure information.

In other words, such a projection in a geographical horizontal plane isable to take into account the attitude of the aircraft and/or the scalefactors of the image sensor 12 as a function of the navigation height.

Without the correct correction tool to implement such a projection, anattitude change could potentially be confused with a change of drift Dof the aircraft.

As indicated above, the module 16 for determining a representative I_(T)information of the route (track) corresponds, for example, to the GPS orGallileo geolocation system of the aircraft, or, according to anotherexample, to the flight management system (FMS).

From the information that is representative of the route and the driftD, the true heading H_(T) may be determined in real time by the module18 for determination of the true heading.

More precisely, the true heading H_(T) is obtained in real time by themodule 18 for determination of the true heading, and that is able tosubtract the drift D from the information that is representative of theI_(T) route, of where: H_(T)=I_(T)−D.

Then, the module 18 for determination of the true heading H_(T) is ableto transmit this true heading H_(T) to at least to the user returnmodule 20 that is able to return to a user, namely to the pilot(s) whenthe device corresponds to an aircraft, information that isrepresentative of this true heading H_(T).

Advantageously, according to a second approach of the present invention,the user return module 20 is a display module capable of recalibrating asynthetic image and/or a conforming symbology with the externalenvironment in the line of sight of the device associated with thedevice, wherein the display module belongs to the group comprising atleast:

-   -   a head-up display module capable of projecting the synthetic        head-up image to the device;    -   a head-up display module capable of projecting the synthetic        mounted head-up image to at least one user of the device;    -   a head-down display module capable of projecting the synthetic        head-down image of the device.

In other words, the present invention makes it possible to correct thedisplay of the synthetic image and/or of the conforming symbology (i.e.recalibration of the heading of the synthetic image is obtained) in thehead-up and/or mounted head-up image and/or even head-down image so thatthe synthetic image of the piloting aid is centered precisely on theaxis of displacement of the aircraft, wherein such an axis ofdisplacement corresponds to the true heading H_(T) obtained previously.

According to an optional aspect, the module for determination of thetrue heading H_(T) is also able to transmit this true heading H_(T) toother avionics modules 26, that are able to use heading information,like the user return module 20. For example, this true heading H_(T) istransmitted to the low-cost AHRS which comprises a magnetometer, inorder to recalibrate it in the event of discrepancy between the magneticheading and this true heading H_(T).

The observation and storage of AHRS D drift over time is alsopotentially useful for allowing predictive maintenance in order todetect a latent failure.

Furthermore, when the image capture implemented by the image sensor 12is no longer available, the memory of the device is able to keep thelast difference E between true heading and the sum of the magneticheading and the magnetic declination, and return it to the module 18 forreal-time determination of the true heading, and able in this case, toadd this difference E to the AHRS magnetic course in order to continueto benefit from a true heading H_(T) (which will drift, as long as theacquisition of the image remains unavailable, in particular duringturning phases, when the intrinsic AHRS magnetic heading error tends toincrease (the measurement of the magnetometer is filtered using agyro-magnetic loop that is sensitive to the errors of the gyroscopeswhen turning).

As an alternative in the case above where the image acquisitionimplemented by the image sensor 12 is no longer available, the module 18for real-time determination of the true heading is suitable forimplementing a Kalman filtering having the AHRS magnetic heading and theheading of the camera (i.e. the image sensor 12 embedded in the camera)as input, and being able to optimize the instantaneous use of these twoinputs of the Kalman filter according to their availability in real timeaccording to an optimization rule defined beforehand. In relation withFIG. 2, the display method implemented by the display system accordingto the present invention is described below.

In general, the method comprises two first steps 32 and 34 for imageacquisition by the image sensor and determination of I_(T) informationthat is representative of the route followed by the aircraft implementedsuccessively (in accordance with any order of execution of these firsttwo steps) or in parallel.

The image acquisition step 32 is followed by an image processing step 36during which the d drift D of the device, namely the aircraft, isdetermined in real time as the successively acquired images areprocessed.

From the drift D and the I_(T) information that are representative ofthe route, a step 38 for determining the true heading H_(T) isimplemented.

Then, according to a step 40, information representative of this captrue H_(T) is returned to the user, namely at least one pilot.

More precisely, the image processing step 36 comprises, according to aparticular aspect, a step 42 of checking (i.e. testing) the type ofimage sensor 12 used.

In particular, if (Y) the image sensor 12 is gyro-stabilized, the imageprocessing step 36 comprises only a step 44 of image analysis byimplementing, for example, a visual odometry technique such as detailedpreviously and specific in order to determine the drift D of theaircraft from the camera drift D_(camera) by change of reference frame(i.e. harmonization correction of reference frames) to go from thecamera reference frame (i.e. reference frame of the image sensor 12) tothe reference frame of the aircraft in order to retranscribe the cameradrift D_(camera) (itself obtained in the camera reference frame) in thereference system of the aircraft.

In the opposite case (N) (i.e. the image sensor 12 is notgyro-stabilized but mounted on a helmet worn by the pilot of theaircraft), the image processing step 36 comprises an additional step 46of correction of the data associated with the acquired images, namely:the data issued directly from the image sensor 12 or the drift data D ofthe aircraft obtained after the step 44 of image analysis.

More precisely, the image data correction step 46 comprises, forexample, a projection step 48 in a geographical horizontal plane takinginto account at least one predetermined datum, namely for example:

-   -   one or more items of information that are representative of the        attitude of the aircraft,    -   one or more items of information that are representative of the        height of the aircraft above the ground,    -   an information that is representative of the visual field of the        image sensor 12.

With regard to the step 40 of return to a user of the device, namely,for example, a pilot of the aircraft, such a step 40 comprises a step 50of recalibrating a synthetic image comprising piloting information inthe axis of movement of the device associated with the true headingH_(T) calculated in real time according to the invention.

For example, such a recalibration 50 makes it possible to readjust thesynthetic image in the axis of the true heading of the aircraft evenbefore seeing the runway during an approach to land.

Such a synthetic image and/or conforming symbology recalibrated in theaxis of the true heading H_(T) is projected by superposition on thepiloting environment of the user, for example according to three steps52, 54, 56 for the possible visualization that is manually activatableand/or automatically selected according to the type of device used,namely a head-up display step 52, a mounted head-up display step 54, ora head-down display step.

It is thus conceivable that the invention allows real-time optimizationof the display of (a) synthetic image(s) and/or of conforming symbologyincluding information of help to the user according to the true headingH_(T) followed by the device, and by avoiding including theimplementation of an expensive inertial unit or by avoiding the use of acomplex database to be enriched and/or updated.

The invention claimed is:
 1. A display system that is included in an airnavigation device, wherein the air navigation device is capable oftraveling along a path between at least two distinct points and using atleast one heading datum, and wherein the display system comprises atleast: an image sensor capable of acquiring environmental images alongthe path traveled by the air navigation device; an image processingmodule configured to determine the drift caused by the air navigationdevice traveling through the air via a drift angle of the air navigationdevice in real time; a geolocation system configured to determineinformation that is representative of the route taken by the airnavigation device; a real-time determination module in communicationwith the image processing module and the geolocation system, wherein thereal-time determination module is configured to determine a true headingfollowed by the air navigation device from the information that isrepresentative of the route taken by the air navigation device and thedrift of the air navigation device determined by the drift angle; and auser return module in communication with the real-time determinationmodule, wherein the user return module is configured to return to a userof the air navigation device information that is representative of thetrue heading.
 2. The display system according to claim 1, wherein theuser return module is a display module configured to recalibrate asynthetic image and/or a conforming symbology in the line of sight ofthe air navigation device associated with the true heading, wherein thedisplay module is selected from the group consisting of: a head-updisplay configured to project the synthetic image to the user of the airnavigation device; a mounted head-up display configured to project thesynthetic image to the user of the air navigation device on the mountedhead-up display worn by the user; a head-down display configured toproject the synthetic image to the user of the air navigation device. 3.The display system according to claim 1, wherein the image processingmodule comprises an image analysis tool for automatically determiningthe drift of the air navigation device by analyzing at least twosuccessive acquired environmental images, wherein the drift is obtainedfrom an angle of displacement of the pixels between one image andanother image, wherein the angle of displacement of the pixels isassociated with the angle between a longitudinal displacement and alateral displacement of the air navigation device.
 4. The display systemaccording to claim 3, wherein the image analysis tool is an opticalodometer.
 5. The display system according to claim 1, wherein the imagesensor is pointed towards the ground.
 6. The display system according toclaim 1, wherein the image sensor has a field of view that isreconfigurable according to the instantaneous roll rate and/orinstantaneous pitch rate of the air navigation device and/or as afunction of the phase of the travel of the air navigation device.
 7. Thedisplay system according to claim 1, wherein the image sensor isintegral with the axes of the air navigation device or isgyro-stabilized.
 8. The display system according to claim 3, wherein theimage processing module further comprises (an) image correction tool(s)that is/are capable of correcting the acquired environmental imagesbefore or after their analysis.
 9. A display method implemented by adisplay system included in an air navigation device that is able totravel a path between at least two distinct points and to use at leastone heading datum, wherein the method comprises: acquiring environmentimages along the path traveled by the air navigation device; processingthe acquired environmental images to determine the drift caused by theair navigation device traveling through the air via a drift angle of theair navigation device in real time; determining information that isrepresentative of the route taken by the air navigation device;determining in real time a true heading followed by the air navigationdevice from the information that is representative of the route taken bythe air navigation device and the drift of the air navigation devicedetermined by the drift angle; and returning to a user of the airnavigation device information that is representative of the determinedtrue heading followed by the air navigation device.
 10. A computerprogram comprising software instructions which, when executed by acomputer, implement the display method according to claim 9.